Room temperature molten salts, such as triethyl ammonium nitrate, have been known for a long time. This product is of no interest except because of the presence of a leaving proton on the cation, limiting the redux or acido-basic stability domain of the compound. Methyl-ethylimidazolium or butyl-pyridinium type compounds, associated to the complex ion [Cl−, xAlCl3] wherein 1<x<2, are also known. These compounds, because of the presence of the aluminium chloride, are powerful Lewis acids, hygroscopic and corrosive because they generate hydrochloric acid in the presence of humidity. Their electrochemical stability domain is also limited by the anodic oxydation of the chloride ion on one side, and by the reduction of the aluminium ion on the other side.
The use of anions usually stables associated to imidazolium or pyridinium type cations has been proposed, but the melting points are relatively high. For example, 1-methyl-3-ethylimidazolium hexafluorophosphate melts at 60° C., and 1,2-dimethyl-3-propylimidazolium hexafluorophosphate melts at 65° C. In addition, these salts, although not hygroscopic, are nevertheless soluble in water and can therefore be hardly prepared by ion exchange in water unless longer alkyl substituents are used, which results in a strong reduction in conductivity and enhanced viscosity.
U.S. Pat. No. 5,827,602 describes salts with a melting point relatively low, with a selection criteria being an anion volume higher than 100 Å3, thus allowing to obtain salts with high conductivity and hydrophobic character. Most representative anions are the bis-trifluoromethanesulfonimidide, that has a calculated volume of 144 Å3 with Hyperchem® program, or the tris-trifluoromethanesulfonylmethylide, which has a volume of 206 Å3.